1. Introduction
Highly efficient nonlinear optical organic materials are of great interest for various applications in optical telecommunication [1, 2, 3, 4] and terahertz wave applications [5, 6, 7]. Due to their almost purely electronic response, organic materials show extremely fast optical nonlinearities compared to their inorganic counterparts, and therefore promise to meet future requirements for ultra-high bandwidth photonic devices (up to 200 GHz an more) for next-generation fiber-optical data transmission networks (ultrafast internet), local-area networks and on-chip applications, as well as sensor applications and field detection. For high-speed second-order nonlinear optical applications, such as electro-optics, second-harmonic generation, optical parametric oscillation, and optical rectification, including terahertz wave generation, a highly asymmetric electronic response of the material to the external electric field is required. A noncentrosymmetric arrangement of the molecules may be induced by spontaneous self-assembly by intermolecular interactions in the crystalline lattice or external poling in a polymer matrix. In crystals the long-term temporal stability of the acentric arrangement of molecules, chromophore density and photochemical stability [8] are significantly superior to those of polymers. Although the strategies leading to large microscopic molecular nonlinearities in a gas phase based on highly extended π-conjugated bridge between electron donor and acceptor groups are well established [1, 2, 9, 10], the desired acentric arrangement of molecules with a high molecular nonlinearity in the solid state is still a very challenging topic.
The state-of-the-art organic nonlinear optical crystals are ionic stilbazolium salts with large electro-optic coefficients of more than 50 pm/V at 1.3 and 1.55 μm, such as DAST (4′-dimethylamino-N-methyl-4-stilbazolium tosylate) [11] and DSTMS (4′-dimethylamino-N-methyl-4-stilbazolium 2,4,6-trimethylbenzenesulfonate) [7]. DAST is presently the only commercially available highly nonlinear organic crystal and exhibits a very large non-resonant nonlinear optical susceptibility χ111(2)(−2ω,ω,ω)=420 pm/V at 1.9 μm and a large electro-optic coefficient r111=53±6 pm/V at 1.3 μm [12]. However, due to the ionic characteristics of stilbazolium salts, their thermal stability is insufficient for applying melt-based crystal growth techniques, which considerably limits the applicability of these crystals for very large scale integrated photonic structures. Furthermore, they have a tendency to form a centrosymmetric hydrated phase in presence of water. Therefore, there is a strong interest to develop new highly nonlinear optical organic crystals with a possibility of melt-based material processing.
Among various crystal engineering approaches [1] to obtain noncentrosymmetric crystal structure, configurationally locked polyene (CLP) crystals developed recently have shown to have high tendency to form acentric crystals with large macroscopic second-order nonlinearities, high thermal stability and the possibility of various melt- and solution-based crystal growth techniques [13, 14, 15]. In the CLP crystals having aliphatic or phenolic OH groups, their main supramolecular interactions are head-to-tail hydrogen bonds of —OH . . . NC— groups at opposite ends of the molecules, leading to large macroscopic nonlinearities [14].